Disadvantages of buying old solid state.

First, on balanced versus unbalanced. Whenever this option comes up, you really have to try it to see if one is better than the other with your components.
 

For instance I have Audio Research equipment. ARC recommends the use of balanced interconnects because the internal designs of both the amp and preamp are balanced. I tried both, I can’t tell the difference. This is not always true, but typically it is not a big difference if any difference can be heard (other than volume…. Which is of no consequence to your system… it will not play louder, it is just the characteristic of the interconnects). My point is, this is not a reason to swap amps… particularly to one older than ten years. It is more likely… the difference in character or age will have a bigger impact.

But if you want a new amp… great idea… focus on the amp / sound character and as you are getting close… just verify it is a balanced design. I would put this at the very end of the list of requirements.

                                              +1 on electrolytic replacement.

     It’s unfortunate, but: some of the older values and/or dimensions can be hard to locate, unless the amp manufacturer stocks them.

     With the way some amps are configured (internal build/PC board mount/etc); that can present a problem.    Especially: when looking for the larger filter caps

@roxy1927 I had to sell off a few beloved 20+ year old Class A/AB SS amps for scrap parts because the boards were deteriorating and no longer available. Gave up after 2-3 board repairs, traces going bad, cracks from heat.  Roll the dice.  Some of the top manufactures back their products after 10+ years, yet rare.  

Over 20yrs old, plan on buying a 2nd one for spare parts or pick a good one backed by someone like Bryston, ARC, Pass, who may actually help you when needed with some kind of guarantee it will last more than a few years more.  If not, simply consider it a throw-away, and this is an option some take.  

Perhaps an anomaly and knock on wood, but my son and I have an Aragon amp bought used in the early 90's, that is still running strong.  Two newer, but still old Aragon multi-channel amps are doing likewise.  Alas, a Phase Linear 400MkII amp bit the dust a decade or so ago, but it was "born" in the late 70's.  However, the last time we fired-up its companion pre-amp, it worked seemingly fine.  And a Pioneer tuner from the late 70's remains going strong. 

Yet to change the subject a bit, computers, smart phones and tablets etc., seem to have more limited lives not due to hardware failures but due to Operating System and program and App advancements.  For example, we have a Zenith IBM compatible computer from the early 80's, that still boots from its huge 30MB hard drive and runs the programs designed for it.   

Amps over time tend to loose their oomph… not fail. It is a very slow process of sonic degradation… unlikely something one would notice day by day. But put in a refurbished or new amp and the difference is striking.